A packaging laminate for a retortable packaging container or carton is known from, for example, international patent application carrying publication number WO97/02140. The prior art packaging laminate has a core layer of paper or paperboard and outer, liquid-tight plastic coatings on both sides of the core layer. The prior art packaging laminate may moreover have a gas barrier disposed between the core layer and one of the two outer plastic coatings in order to impart to the packaging laminate tightness properties against gases, in particular oxygen gas, if such is desirable.
The prior art packaging laminate according to WO97/02140 is produced by a conversion process in which a paper or paperboard web is coated on its one side with a first, outer plastic coating, and the other side of the web is provided with an aluminium foil (Alifoil) serving as gas barrier with the aid of a lamination layer which is extruded between the web and the Alifoil. The Alifoil is covered with a second, outer plastic coating by an outer lamination layer which is extruded between the web and the second plastic coating. In a subsequent printing operation, the web is provided with any optional decorative artwork of printing ink which is printed in repeated patterns on the first plastic coating of the web and immediately thereafter is dried or cured. In a later forming operation, the web is reformed into a tube in that both longitudinal edges of the web are united with one another in an overlap joint or seam at the same time as the tube is flat-folded and divided by transverse incisions in the regions between the recurring patterns of printing ink, for the formation of individual, flat-folded tubular packaging blanks.
From the flat-folded, tubular packaging blanks, the finished retortable packaging containers are produced with the aid of packaging machines of the type which form, fill and seal finished packages. The flat-folded packaging blank is first raised to an open tubular packaging carton which is sealed at its one end by a fold-forming and sealing operation. The packaging carton is filled with the desired contents, e.g. a food, through its other open end which, after the filling operation, is closed by a further fold-forming and sealing operation. The thus filled and sealed packaging container is then ready for a shelf-life extending heat treatment which is normally carried out in a retort.
A suitable heat treatment, or so-called retorting, is described in international patent application carrying publication number WO98/16431. In general, the packaging container is heated to a temperature within the range of approx. 70–130° C. The time for the complete treatment, including the time for heating and the time for cooling, should be sufficient to impart to the packed food a suitable combination of high Fo-value and low Co-value, as a person skilled in the art will be aware.
In such instance, the expression “Fo-value” relates to a corresponding time (min) which the food should be kept at a reference temperature (121° C.) for obtaining the same level of sterility, which the expression “Co-value” relates to a cooking value and is a corresponding time (min) which the food should be kept at a reference temperature (100° C.) in order to obtain the same level of cooking effect in all of its component ingredients.
A retortable packaging container or carton of the prior art packaging laminate is per se sufficiently mechanically configurationally stable to withstand the powerful stresses to which it is subjected during a heat treatment or retorting of the above-described type. However, it not seldom occurs that the printing ink decorative artwork of the packaging container may be affected and deteriorate and even totally loose its distinctive printing quality if it is subjected to extreme humidity and temperature stresses during the heat treatment.
Since a pristine, distinct printing decorative artwork is, for several obvious reasons, desirable and sought-for, not least in order to impart to the packaging container an attractive appearance which is readily recognised and appreciated on the part of the consumers, numerous attempts have been made to obviate the problem inherent in undesired affect on the printing ink decorative artwork in connection with the prior art retortable packaging containers or cartons.
One such attempt takes as its point of departure protecting the printed decorative artwork with the aid of a prefabricated plastic film which is applied on the plastic coating of the prior art packaging laminate provided with the decorative artwork in direct contact with the subjacent printing ink decorative artwork. Practical trials have, however, shown that not only is it difficult to achieve a desirable requisite adhesion between the printing ink and the additional plastic film, but also the plastic film has shown a tendency to loosen from the subjacent plastic coating by shearing in connection with a subsequent heat treatment or retorting of the packaging carton.